Integrated circuit storage element having low power data retention and method therefor

ABSTRACT

A storage element ( 10 ) includes a first latch ( 12 ) and a second latch ( 14 ). The first latch ( 12 ) is coupled to a first power supply voltage terminal for receiving a first power supply voltage. The second latch ( 14 ) is coupled to a second power supply voltage terminal. The second power supply voltage terminal for receiving a second power supply voltage that is lower than the first power supply voltage. During a normal mode of operation, the second power supply voltage is not provided to the second latch. During a low power mode of operation data is transferred from the first latch to the second latch and the first latch is powered down. The data is retained by the second latch while in low power mode.

RELATED APPLICATION

A related, copending application is entitled “Integrated Circuit Having a Low Power Mode and Method Therefor”, by Ramaraju et al., U.S. application Ser. No. 11/065,796, is assigned to the assignee hereof, and filed concurrently herewith.

FIELD OF THE INVENTION

This invention relates generally to integrated circuits, and more particularly to a storage element having low power data retention and a method therefor.

RELATED ART

Integrated circuits are being manufactured with increasingly more transistors. As the transistor count goes up, static leakage power also increases. One way to reduce the static leakage power is to operate the integrated circuit with a lower power supply voltage. Another way to reduce the static leakage power is to manufacture the integrated circuit with relatively higher threshold voltages. However, both of these techniques may lower the operating speed of the integrated circuit.

Integrated circuits are commonly designed having a lower power, or idle mode, of operation to reduce static power consumption during times of little or no activity. It may be important to retain the logic state of certain storage elements so that the integrated circuit can be returned to a normal operating mode without loss of information. One technique that has been used to accomplish this is to store the information in a separate memory array before entering the low power mode. This can be complex to implement when there is a large amount of information. Another technique is to remove the power supply from portions of the integrated circuit while leaving other portions powered up. This reduces some leakage power but not all.

Therefore, it would be desirable to further reduce static power consumption during a low power mode of operation while maintaining transistor switching speed during a normal mode of operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to like elements and in which:

FIG. 1 illustrates, in schematic diagram form, an integrated circuit storage element in accordance with one embodiment of the present invention.

FIG. 2 illustrates a timing diagram of various voltages in the integrated circuit storage element of FIG. 1.

FIG. 3 illustrates, in schematic diagram form, an integrated circuit storage element in accordance with another embodiment of the present invention.

FIG. 4 illustrates, in block diagram form, a portion of an integrated circuit in accordance with an embodiment of the present invention.

FIG. 5 illustrates a timing diagram of various supply voltages is accordance with another embodiment of the present invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

Generally, in one embodiment, the present invention provides an integrated circuit storage element having a first latch and second latch having input/output (I/O) terminals coupled to the I/O terminals of the first latch. The first latch is powered by a first power supply voltage and the second latch is power by a second power supply voltage. The second voltage is provided at a lower potential than the first power supply voltage. During a normal mode of operation of the integrated circuit, the first power supply voltage is provided to the first latch and the second, lower, power supply voltage is removed from the second latch. When the integrated circuit is powered down, or operated in a reduced or lower power mode, the second power supply is first provided to the second latch. While the first power supply voltage is being provided to the first latch and the second power supply voltage is being provided to the second latch, data stored in the first latch is transferred automatically to the second latch. After the data is transferred, the first power supply is removed from the first latch. Data is retained in the second latch. To reduce leakage current in the second latch during the lower power mode, the transistors of the second latch may be optionally manufactured using transistors having a higher threshold voltage (V_(T)).

Conversely, when the integrated circuit is to be returned to a normal operating mode, the first power supply is first reapplied to the first latch. The second latch continues to receive the second power supply voltage while the data is automatically transferred back to the first latch. After the data is transferred from the second latch back to the first latch, the second power supply voltage is removed from the second latch after a predetermined amount of time sufficient to allow the data to be transferred.

Because the second latch is not used during normal operation, it does not affect the operating speed of the integrated circuit. By operating the second latch with a second, lower, power supply voltage as described herein, an integrated circuit can have high speed operation during a normal operating mode while also having relatively lower static power consumption during a low power operating mode.

FIG. 1 illustrates, in schematic diagram form, an integrated circuit storage element 10 in accordance with one embodiment of the present invention. Integrated circuit storage element 10 is preferably implemented on an integrated circuit using complementary metal oxide semiconductor (CMOS) transistors. However, in other embodiments, the storage element 10 may be implemented using different types of transistors. Storage element 10 includes latches 12 and 14, inverter 16, and N-channel transistors 18, 20, 38, 40, 42, and 44. Latch 12 includes P-channel transistors 22 and 26 and N-channel transistors 24 and 28. Latch 14 includes P-channel transistors 30 and 34 and N-channel transistors 32 and 36.

In one embodiment, as illustrated in FIG. 1, latches 12 and 14 each comprise a pair of cross-coupled inverters. Transistors 22 and 24 are connected together to form one inverter in latch 12, and transistors 26 and 28 are connected together to form the other inverter. Likewise, transistors 30 and 32 are connected together to form one inverter in latch 14, and transistors 34 and 36 are connected together to form the other inverter. Nodes 23 and 25 form both I/O terminals and storage nodes for latch 12, and nodes 31 and 35 form both I/O terminals and storage nodes for latch 14. Latch 12 has a power supply voltage terminal labeled “V_(DD)” for receiving a first power supply voltage. Latch 14 has a power supply voltage terminal labeled “LV_(DD)” for receiving a second power supply voltage, where the second power supply voltage is lower than the first power supply voltage. For example, V_(DD) may be about 1.2 volts and LV_(DD) may be about 0.6 volts. In other embodiments, the power supply voltages may be different. Also, in the illustrated embodiment, the power supply voltages V_(DD) and LV_(DD) are both provided from a source external to the integrated circuit. However, in other embodiments, one skilled in the art will recognize that the two power supply voltages V_(DD) and LV_(DD) may be derived from a single external source and stepped up or down, as desired on the integrated circuit.

N-channel transistor 18 has a drain (current electrode) connected to node 23, a source (current electrode) connected to ground (V_(SS)), and a gate (control electrode) connected to receive an input data signal labeled “D_(IN)”. N-channel transistor 20 has a drain connected to node 25, a source connected to V_(SS), and a gate connected to receive an input data signal labeled “D_(IN)*”. Note that the asterisk (*) after the signal name indicates that the signal is a logical complement of a signal having the same name but lacking the asterisk.

N-channel transistor 38 has a drain connected to node 23, a source connected to VSS, and a gate connected to node 31 of latch 14. N-channel transistor 42 has a drain connected to node 25, a source connected to V_(SS), and a gate connected to node 35 of latch 14. N-channel transistor 40 has a drain connected to node 31, a gate connected to node 23, and a source connected to V_(SS). N-channel transistor 44 has a drain connected to node 35, a gate connected node 25, and a source connected to V_(SS). Note that in the illustrated embodiment, V_(SS) is at zero volts, or ground. In other embodiments, V_(SS) may be at a different potential.

FIG. 2 illustrates a timing diagram of various voltages in the integrated circuit storage element 10 of FIG. 1. The operation of integrated circuit storage element 10 will be described with reference to both FIG. 1 and FIG. 2.

In operation, when integrated circuit storage element 10 is operating in a normal mode of operation, as illustrated in FIG. 2 between times T0 and T1, latch 12 operates in a conventional manner. V_(DD) is provided to latch 12 at the normal operating voltage and LV_(DD) is not provided to latch 14. In FIG. 2, LV_(DD) is at zero volts. Differential data input signals D_(IN)/D_(IN)* are provided to the gates of transistors 18 and 20. A differential logic state is stored at nodes 23 and 25 corresponding to the logic states of D_(IN)/D_(IN)*. Note that only the logic state on node 23 is shown in FIG. 2, however because signals D_(IN)/D_(IN)* are differential, nodes 23 and 25 store opposite logic states When the voltage of node 23 is a logic high or at V_(DD), the voltage of node 25 (not shown) would be at a logic low, or at zero volts. Note that differential data signals are provided in the illustrated embodiments, however, in other embodiments, a single-ended input data signal may be used. Also note that transistors 18 and 20 may not be present in other embodiments, and a data signal may be directly inputted, or received, from the nodes 23 and 25.

Between times T1 and T2, FIG. 2 illustrates transitioning from normal mode to low power mode. To transition from normal operating mode to low power operating mode, the second power supply voltage LV_(DD) is first provided to latch 14 at time T1 while V_(DD) is still at a normal operating voltage. While LV_(DD) is being provided to latch 14 and V_(DD) is provided to latch 12, data stored in the latch 12 is transferred automatically to latch 14 through transistors 40 and 44. For example, if node 23 is storing a logic high, node 25 will be at a logic low. A logic high voltage will be provided to the gate of transistor 40, causing transistor 40 to be conductive. Likewise, a logic low voltage at the gate of transistor 44 will cause transistor 44 to be substantially non-conductive. A voltage on node 31 will be reduced to about V_(SS), or ground, and a voltage on node 35 (not shown in FIG. 2) will be at about LV_(DD). After a predetermine length of time sufficient to allow the data to be transferred, V_(DD) is removed from latch 12 at time T2, and the logic state of the data is retained in latch 14. To further reduce leakage current in the second latch during the low power mode, the transistors of the second latch may be optionally manufactured using transistors having a higher threshold voltage (V_(T)).

To transition back to the normal mode of operation, V_(DD) is first reapplied to the power supply voltage terminal of latch 12 at time T3. If a logic low voltage is being stored on node 31 and a logic high voltage is stored on node 35, transistor 42 will be conductive, causing node 25 of latch 12 to be reduced to V_(SS). Transistor 38 will be substantially non-conductive, allowing a voltage on node 23 to be increased to about V_(DD). Latch 14 continues to receive LV_(DD) while the data is automatically transferred back to latch 12. After the data is transferred from latch 14 back to latch 12, LV_(DD) is removed from latch 14 after a time period sufficient to allow the data to be transferred at time T4. Preferably, a data input signal D_(IN)/D_(IN)* is not present at the gates of transistors 18 and 20 during the time data is transferred from latch 14 to latch 12. Also, when transitioning back to normal mode, a ramp rate of the power supply voltage V_(DD) may be controlled to make it easier for the stored logic state of nodes 31 and 35 of latch 14 to toggle nodes 23 and 25 of latch 12.

FIG. 3 illustrates, in schematic diagram form, an integrated circuit storage element 50 in accordance with another embodiment of the present invention. Note that for the purpose of clarity and brevity, like or similar elements in FIG. 1 and FIG. 3 have the same reference numbers. Storage element 50 is different from storage element 10 in that latch 14 includes additional P-channel transistors 52 and 54 and additional N-channel transistors 56 and 58. The additional transistors are optional and function to disconnect LV_(DD) from latch 14 and to cause nodes 31 and 35 to be coupled to V_(SS) in response to a disable signal labeled “DISABLE” being asserted as a logic high voltage. P-channel transistors 52 and 54 each have their sources connected to LV_(DD), their gates coupled to receive signal DISABLE, and their drains connected to the sources of transistors 30 and 34. N-channel transistors 56 and 58 are coupled in parallel with transistors 32 and 36, respectively. The gates of N-channel transistors 56 and 58 are coupled to receive signal DISABLE.

In operation, during normal mode, storage element 50 functions similarly to storage element 10, except that signal DISABLE and transistors 52, 54, 56, and 58 are shown as one way to disconnect the power supply voltage from LV_(DD). When storage element 50 is operating in normal mode, signal DISABLE is asserted as a logic high and transistors 56 and 58 are conductive and transistors 52 and 54 are substantially non-conductive. This causes LV_(DD) to be disconnected from the sources of transistors 30 and 34 and causes nodes 31 and 35 of latch 14 to be at ground potential. During low power mode, signal DISABLE is asserted as a logic low, causing transistors 52 and 54 to be conductive, and coupling LV_(DD) to the sources of transistors 30 and 34. Transistors 56 and 58 are substantially non-conductive, allowing the logic states of nodes 23 and 25 of latch 12 to change the logic states of nodes 31 and 35 of latch 14.

FIG. 4 illustrates, in block diagram form, a portion of an integrated circuit 70 in accordance with an embodiment of the present invention. Integrated circuit 70 may be, for example, a microprocessor, or memory, or any type of integrated circuit that has a normal operating mode and a low power operating mode. Integrated circuit 70 includes a normal mode power supply bus 72 and a low power mode power supply bus 74. The storage element 10, illustrated in FIG. 1, is coupled to both power supply buses 72 and 74 for receiving power a supply voltage V_(DD) and power supply voltage LV_(DD). The power supply voltage V_(DD) is a normal power supply voltage that is applied to the integrated circuit during normal circuit operation. The power supply voltage LV_(DD) is a power supply voltage provided to certain logic or storage elements of integrated circuit 70 that are necessary for preserving a state of integrated circuit 70 during a low power mode, such as for example, a “sleep” mode. In one embodiment, the power supply buses 72 and 74 are laid out on integrated circuit 70 as metal conductors. The power supply bus 72 may be part of a power supply grid that provides V_(DD) to circuits all over the integrated circuit 70. The power supply bus 74 provides LV_(DD) to only those parts of integrated circuit 70 that require LV_(DD) during a low power mode and is therefore relatively more lightly loaded than power supply bus 72.

A logic block 76 is coupled to normal mode power supply bus 72 for receiving power supply voltage V_(DD). The logic block 76 illustrates various circuits in integrated circuit 70 that are powered during the normal operating mode but are not powered during the low power mode. The logic block 76 is coupled to receive data signals D_(OUT) from storage element 10 and to provide signals to a storage element 77.

Storage elements 10 and 77 are representative of a plurality of storage elements in integrated circuit 70 that store information such as data, control, address and other signals necessary during normal operation. Also, the information stored by the plurality of storage elements represented by storage elements 10 and 77 must be retained during a low power mode in order for the integrated circuit 70 to operate properly when returning to a normal operating mode. Storage element 77 includes a first portion 78 and a second portion 80. The first portion 78 is powered by V_(DD) and operates during a normal operating mode. The second portion 80 is powered by power supply bus 74. Storage element 77 may be the same as storage element 10 as illustrated in FIG. 1 or FIG. 3, or storage element 77 may be a different type of storage device having a latch is that only used during normal mode, and a latch that is only used during low power mode.

Latch 77 is coupled to receive signals from logic 76 and provides signals to a memory 84. Memory 84 may be any type of random access memory, such as for example, a static random access memory (SRAM), a dynamic random access memory (DRAM), or a non-volatile memory such as flash. Memory 84 is coupled to power supply bus 72 and may optionally be coupled to power supply bus 74 so that memory 84 may operate in low power mode if necessary.

By selectively providing a lower power supply voltage (LV_(DD)) to certain storage elements of an integrated circuit during a low power mode, the normal power supply voltage (V_(DD)) may be turned off. The lower power supply voltage LV_(DD) is high enough to allow the storage elements to retain their states and low enough that leakage current is minimized. During a normal operating mode, the low power supply voltage can be turned off. Note that in other embodiments, additional low power mode power supply buses LV_(DD) may be implemented. Also, the additional low power mode power supply buses may provide different voltages as described below.

At present, an integrated circuit may include various circuit modules that cooperate to perform a function that once required several integrated circuits. The modules of a “system-on-a-chip” (SOC) integrated circuit may include circuits that are implemented with different transistor types that display different leakage characteristics. FIG. 5 illustrates a timing diagram of various supply voltages is accordance with another embodiment of the present invention that may be used on a SOC type of application.

In FIG. 5, a voltage labeled V_(DD) is a normal power supply voltage for the SOC. The power supply voltage V_(DD) may be, for example, 1.2 volts. A voltage labeled LV_(DD1) is a lower power supply voltage that is provided to a module that requires, for example, 0.7 volts during a low power mode. A voltage labeled LV_(DD2) is another lower power supply voltage that is provided to a different module that requires, for example, 0.6 volts during a low power mode. If storage element 10 (FIG. 1) is implemented in a first module that requires LV_(DD1), then a latch 14 in the first module would be powered by LV_(DD1) and a latch 12 in the first module would be powered by V_(DD). Likewise, if storage element 10 is implemented in a second module that requires LV_(DD2) during low power mode, then latch 14 in the second module would be powered by LV_(DD2).

For the SOC example above, during a normal mode of operation as indicated in FIG. 5 between times T0 and T1, V_(DD) is provide to the SOC and LV_(DD1) and LV_(DD2) are at zero volts. Note that the voltages presented are for discussion purposes only and may be different in other embodiments.

At time T1, a low power mode, such as sleep mode, is entered and V_(DD) transitions to an intermediate voltage labeled V1, where voltage V1 is equal to approximately the voltage of LV_(DD1). At time T2, LV_(DD1) rises to voltage V1 and the logic state of a latch 12 is transferred to a latch 14 while both V_(DD) and LV_(DD1) are at V1. At time T3, V_(DD) drops to a voltage V2, where voltage V2 is equal to approximately the voltage LV_(DD2). The logic state of another latch 12 is transferred to another latch 14. At time T4, the voltage of V_(DD) is reduced to zero volts and remains at zero volts until the integrated circuit is returned to a normal operating mode. At time T4, the transition to low power mode is complete. The states of the low power latches 14 are retained as discussed above in the discussion of FIG. 1.

While in the low power mode, there is very little or no leakage current from the circuits powered only by V_(DD) because V_(DD) has been reduced to zero voltage. The only leakage current is from the circuits powered by LV_(DD1) and LV_(DD2), but because LV_(DD1) and LV_(DD2) are provided at a lower voltage than V_(DD), the leakage current is minimized. At discussed above in the description of FIG. 1, because latch 14 of storage element 10 is not in a critical speed path of the integrated circuit, the transistors may be implemented to have a higher threshold voltage, thus reducing leakage current even further.

At time T5, the normal operating mode is re-entered and V_(DD) begins to ramp up to voltage V2. The logic state of latch 14 of the second module is transferred to latch 12 of the second module while V_(DD) and LV_(DD2) are about equal. At time T6, LV_(DD2) is reduced to zero volts. At time T7, V_(DD) increases to voltage V1 and the logic state of latch 14 of the first module is transferred to latch 12 of the first module while V_(DD1) and LV_(DD1) are approximately equal. At time T8, LV_(DD1) is reduced to zero volts. At time T8, the voltage of V_(DD) increases to the normal operating voltage and a normal operating mode is resumed.

While the invention has been described in the context of a preferred embodiment, it will be apparent to those skilled in the art that the present invention may be modified in numerous ways and may assume many embodiments other than that specifically set out and described above.

Accordingly, it is intended by the appended claims to cover all modifications of the invention which fall within the true scope of the invention.

Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. As used herein, the terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. 

1. A storage element comprising: a first latch having first and second input/output (I/O) terminals, and a first power supply voltage terminal, the first power supply voltage terminal for receiving a first power supply voltage; and a second latch having third and fourth I/O terminals coupled to the first and second I/O terminals, and a second power supply voltage terminal, the second power supply voltage terminal for receiving a second power supply voltage; wherein the second power supply voltage is lower than the first power supply voltage, wherein the second power supply voltage is not provided to the second power supply voltage terminal when the first power supply voltage is being provided to the first power supply voltage terminal, and wherein the first power supply voltage is not provided to the first power supply voltage terminal when the second power supply voltage is being provided to the second power supply voltage terminal.
 2. The storage element of claim 1, further comprising: a first transistor having a first current electrode coupled to the first I/O terminal, a control electrode coupled to the third I/O terminal, and a second current electrode coupled to ground; and a second transistor having a first current electrode coupled to the second I/O terminal, a control electrode coupled the fourth I/O terminal, and a second current electrode coupled to ground.
 3. The storage element of claim 2, further comprising: a third transistor having a first current electrode coupled to the third I/O terminal, a control electrode coupled to the first I/O terminal, and a second current electrode coupled to ground; and a fourth transistor having a first current electrode coupled to the fourth I/O terminal, a control electrode coupled to the second I/O terminal, and a second current electrode coupled to ground.
 4. The storage element of claim 3, wherein the second latch and the first, second, third, and fourth transistors are formed using a high threshold voltage complementary metal oxide semiconductor (CMOS) process.
 5. The storage element of claim 1 wherein there is a time period when both the first and second power supply voltages are provided to the first and second latches at the same time.
 6. The storage element of claim 5 wherein data transfer occurs between the first and second latches during the time period.
 7. The storage element of claim 1, wherein each of the first and second latches are formed using a pair of cross-coupled inverters.
 8. The storage element of claim 1, further comprising: a first transistor having a first current electrode coupled to the first I/O terminal, a control electrode for receiving a first input signal, and a second current electrode coupled to ground; and a second transistor having a first current electrode coupled to the second I/O terminal, a control electrode for receiving a second input signal, and a second current electrode coupled to ground.
 9. The storage element of claim 1, further comprising a decoupling circuit for decoupling the second latch from the second power supply voltage terminal from the second power supply voltage in response to a disable signal being asserted.
 10. The storage element of claim 9, wherein the decoupling circuit further comprises a coupling circuit for coupling the third and fourth I/O terminals to ground in response to the disable signal being asserted.
 11. The storage element of claim 9, wherein the disable signal is not asserted when an integrated circuit comprising the storage element is operating in a low power mode, and the disable signal is asserted when the integrated circuit is operating in a normal mode.
 12. A storage element comprising: a first latch having first and second input/output (I/O) terminals, and a first power supply voltage terminal, the first power supply voltage terminal for receiving a first power supply voltage; a second latch having third and fourth I/O terminals coupled to the first and second I/O terminals, and a second power supply voltage terminal, the second power supply voltage terminal for receiving a second power supply voltage; a first transistor having a first current electrode coupled to the first I/O terminal, a control electrode coupled to the third I/O terminal, and a second current electrode coupled to ground; a third transistor having a first current electrode coupled to the second I/O terminal, a control electrode coupled the fourth I/O terminal, and a second current electrode coupled to ground; a fourth transistor having a first current electrode coupled to the third I/O terminal, a control electrode coupled to the first I/O terminal, and a second current electrode coupled to ground; and a fourth transistor having a first current electrode coupled to the fourth I/O terminal, a control electrode coupled to the second I/O terminal, and a second current electrode coupled to ground; wherein the second power supply voltage is lower than the first power supply voltage, wherein the second power supply voltage is not provided to the second power supply voltage terminal when the first power supply voltage is being provided to the first power supply voltage terminal, and wherein the first power supply voltage is not provided to the first power supply voltage terminal when the second power supply voltage is being provided to the second power supply voltage terminal.
 13. The storage element of claim 12, wherein the second latch and the first, second, third, and fourth transistors are formed using a high threshold voltage complementary metal oxide semiconductor (CMOS) process.
 14. The storage element of claim 12 wherein there is a time period when both the first and second power supply voltages are provided to the first and second latches at the same time and wherein data transfer occurs between the first and second latches during the time period.
 15. The storage element of claim 12, further comprising: a first transistor having a first current electrode coupled to the first I/O terminal, a control electrode for receiving a first input signal, and a second current electrode coupled to ground; and a second transistor having a first current electrode coupled to the second I/O terminal, a control electrode for receiving a second input signal, and a second current electrode coupled to ground.
 16. The storage element of claim 12, further comprising a decoupling circuit for decoupling the second latch from the second power supply voltage terminal from the second power supply voltage in response to a disable signal being asserted.
 17. The storage element of claim 16, wherein the decoupling circuit further comprises a coupling circuit for coupling the third and fourth I/O terminals to ground in response to the disable signal being asserted.
 18. The storage element of claim 16, wherein the disable signal is not asserted when an integrated circuit comprising the storage element is operating in a low power mode, and the disable signal is asserted when the integrated circuit is operating in a normal mode.
 19. In an integrated circuit comprising a storage element, the storage element comprising a first latch having first and second input/output (I/O) terminals and a first power supply voltage terminal, a second latch having third and fourth I/O terminals coupled to the first and second I/O terminals and a second power supply voltage terminal, the method comprising: providing a first power supply voltage to the first power supply voltage terminal; providing a first input signal having a first logic state to the first I/O terminal; storing the first logic state in the first latch; providing a second power supply voltage to the second power supply voltage terminal, the second power supply voltage being having a lower potential than the first power supply voltage; transmitting the first logic state from the first latch to the second latch automatically in response to the first power supply voltage being provided to the first power supply voltage terminal and the second power supply voltage being provided to the second power supply voltage terminal; and removing the first power supply voltage from the first power supply voltage terminal.
 20. The method of claim 19, further comprising: reapplying the first power supply voltage to the first power supply voltage terminal; transmitting the first logic state from the second latch to the first latch automatically in response to the first power supply voltage being provided to the first power supply voltage terminal and the second power supply voltage being provided to the second power supply voltage terminal; and removing the second power supply voltage from the second power supply voltage terminal. 